Cationic polymers or resins exhibiting thermosetting properties are useful for increasing the wet strength of paper products and reducing paper “creep” while the paper is wet. One well-known class of such polymers is the polyamidoamine-epihalohydrin (PAE) resins. In the absence of such cationic wet strengthening resins, paper normally retains only about 3% to 5% of its strength after being wetted with water. However, paper made or treated with a cationic wet strengthening resin, such as a PAE resin, generally retains at least 10%-50% of its strength when wet. As such, these resins are in wide use.
As is well-known, the PAE resins can be made by the reaction of an epihalohydrin, usually epichlorohydrin, with a polyamidoamine (alternatively referred to as a polyaminoamide, a polyamidopolyamine, a polyaminopolyamide, a polyaminamide and the like). The reaction is typically performed in an aqueous solution under a basic condition (e.g., at a pH between about 7 to about 11) often followed by diluting the reaction product to a relatively low solids content.
Such PAE resins also can be blended with other ionic or non-ionic polymers, such as but not limited to polyvinyl alcohol (PVA) polymers, polyethylene oxide (PEO) polymers, hydroxyethylcelluloses, poly diallyldimethyl ammonium chloride (DADMAC) polymers and the like, for wet strengthening applications. These resin or polymer blends also tend to exhibit a limited storage stability depending in part on the component ratios in the blends.
Historically, due to the high reactivity of such cationic polymers and particularly the widely used PAE resins, the solids contents of the final resin solutions have been diluted to and maintained at about 10 to 15% in order to prevent premature gelation of the resin upon standing (storage) at room temperature. Such gelation obviously contributes to a loss of wet strength efficiency and often renders the resin totally unusable. Thus, for the most part, such cationic resins and the PAE resins in particular have been shipped and stored in a relatively dilute form to paper mills where the resins are ultimately used. This practice increases costs to the mill since, in effect, the mill is paying shipping costs for transporting water and added storage costs because of the higher volume of material being stored.
Given these circumstances, the art has long recognized the benefit that could be obtained by increasing the solids content of aqueous cationic thermosetting polymers, such as the noted PAE resins. Unfortunately, untreated cationic thermosetting polymers, such as the PAE resins, stored at higher solids concentrations are more prone to experience a gradual increase in viscosity to gelation. The inherent viscosity increase places a time limit on how long such resins can be stored before they must be used. Stability is generally judged by the time between the preparation of the polymer or resin and the time it gels (i.e., the viscosity increase is so great that the resin becomes non-functional).
In one approach for improving the storage stability of PAE resins, such resins have been contacted with an acid to stabilize the product. See, for example, U.S. Pat. Nos. 3,311,594, 3,197,427, 3,442,754 and 4,853,431. Ordinarily, the higher that the solids content in the resin solution is, the lower the pH must be maintained in order to provide for suitable storage stability of the resin, i.e., to prevent the material from prematurely forming a gel. Reducing pH to improve stability, however, has its limits since increasingly lowering the pH exacerbates resin hydrolysis and thus reduces the wet strengthening effectiveness of the resin, especially cationic PAE resins.
In another stabilization approach. Keim in U.S. Pat. No. 3,240,761, for example, includes a quaternizing agent such as an alkyl halide during the latter stages of the polyamide-epichlorohydrin reaction. Coscia U.S. Pat. No. 3,259,600 describes adding a stoichiometric excess of certain metal complexing salts to the aqueous resin solution in order to form metal coordination complexes which purportedly enhance resin stability. Earle, in U.S. Pat. No. 3,352,833, describes using an acidic hydrogen halide such as hydrochloric acid, to stabilize the epichlorohydrin moiety of such aqueous resins purportedly without reducing wet strength efficiency by forming the corresponding aminochlorohydrin hydrochloride. Keim, in U.S. Pat. No. 3,227,671, describes adding a small quantity of formaldehyde to the PAE resin following its synthesis and before the resin is cooled to improve its storage stability.
In yet another approach alleged to produce a high solids PAE resin that is stable for up to four weeks, U.S. Pat. No. 6,222,006 reacts epichlorohydrin with an end-capped polyaminamide (polyamidoamine). As described, the polyaminamide is end-capped with hydrocarbon radical(s) by including a monoacid or monoester (or alternatively some functional equivalent chain terminator) in the synthesis of the polyaminamide.
While these approaches have had some success in improving the stability of cationic wet strengthening polymers and especially PAE wet strengthening resins, there remains much room for further improvement. Accordingly, the art continues to search for alternative ways to stabilize such water-soluble, cationic wet strengthening polymers, such as the cationic polyamidoamine-epihalohydrin (PAE) resins, with the goal of permitting such polymers to be maintained in solution at a relatively higher solids content without the need to lower excessively the pH of the solution and risk resin hydrolysis. In particular, a procedure which stabilizes a high solids content aqueous solution of a cationic polymer resin, such as a PAE resin, against gelation, while at the same time providing stability against a significant loss in solution viscosity would constitute a significant improvement.